1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
The coverage area of a wireless communication system is typically divided into a number of cells, which may be grouped into one or more radio access networks. The coverage area of each cell in the wireless network is typically limited by the propagation loss of radio signals transmitted by base stations that provide coverage to the cell. Thus, the coverage area of each cell is determined by the location and the transmit power of the base station, as well as the topology of the cell and the location of any interfering objects. For example, the coverage area of a cell may be reduced if a building or a mountain is present near the base station. The boundaries of the cells are not rigidly defined and may vary with time. Thus, coverage areas may overlap such that multiple base stations may provide coverage to the overlapping regions, although the strength of the signal provided within the overlapping regions may be different for the different base stations.
Mobile units located in the coverage area of a cell may access the wireless communications system by establishing a wireless communication link with the base station associated to that cell. This is often referred to as the air interface. As discussed above, a mobile unit may be located in a region served by more than one base station. The mobile unit may then select the base station having the lowest propagation path loss (or highest channel quality) as the serving base station. Roaming mobile units may travel through regions served by numerous base stations. Accordingly, a roaming mobile unit may handover from one base station to another as it enters and leaves cells served by different base stations. Stationary mobile units may also handover from one base station to another if the propagation path loss (or channel quality) associated with the base stations varies over time.
Handover typically refers to a category of procedures that may be used to support mobility for mobile units in a cellular wireless network. For example, handover techniques may be used to allocate different radio resources for the existing connection as the mobile unit is handed over from one cell to another cell. When performing handover from one cell to another cell this generally means that different radio resources need to be allocated for this connection. In Code Division Multiple Access (CDMA) networks like the Universal Mobile Telecommunication System (UMTS), the basic radio resources are carrier frequencies, transmit power, and spreading codes. Due to the wide bandwidth of the carriers in UMTS systems, most network operators have only two or three carriers available for the entire network and, hence, carrier frequencies are often re-used in every base station. Many mobiles then need to share the same frequency band. The signals transmitted between the cells and mobile units are therefore distinguished by their spreading code. Signal separation using the spreading codes is maintained when moving between cells. Soft and/or softer handover techniques that permit a mobile unit to communicate concurrently with multiple base stations are typically applied when the mobiles travel through the network using the same frequency in every cell. Due to the fact that frequencies are not changed, soft and/or softer handover is also known as intra-frequency handover in UMTS.
Mobile units may also be handed off between cells that operate on different frequencies. For example, the Third generation Partnership Project (3GPP) standards for application in UMTS networks specify an inter-frequency handover. As mentioned above, most operators only have a few carrier frequencies available for their networks. One carrier is typically used to provide continuous coverage and basic services and the additional carriers are made available when needed, e.g., to provide coverage to larger network areas or to selected hotspots. Mobile units may therefore need to change carrier frequencies, i.e. to perform an inter-frequency handover within the multi-carrier system. Inter-frequency handovers typically occur when the load conditions on the two carriers change, e.g., the cells on one carrier might become overloaded. Inter-frequency handovers may also be used when the coverage areas of additional carriers are limited, e.g., the second and/or third carrier might be only used in hotspot areas. For example, mobile units using a hotspot carrier may be handed over to other carriers if traveling towards the border of the hotspot.
In some cases, mobile units may even be handed off between base stations in different wireless communication systems. For example, the 3GPP standards also specify inter-system (or inter-Radio Access Technology) handover for application in UMTS networks. One example of a handover between different radio access technologies (RAT) would be handover between a UMTS network and a network that operates according to the Global System for Mobile communication (GSM) standard. Many network operators have deployed both a UMTS network and a GSM network. The GSM networks have been deployed longer than UMTS networks and therefore the GSM networks most often offer nearly continuous nationwide coverage. In contrast, UMTS networks have (and are expected to continue to have) numerous coverage holes as operators concentrate on offering the new UMTS services in areas of relatively high population density. Inter-RAT handover is therefore a very critical feature in mixed technology networks. For example, mobile units traveling outside the UMTS coverage area will be dropped if they do not perform a handover to the GSM access technology, which may also cause customer satisfaction to fall.
Conventional handover techniques, whether soft, inter-frequency, or inter-RAT, rely on measurements performed by the mobile unit and/or the network. For example, each mobile unit may perform measurements based on signals transmitted between the mobile units and base stations that are currently connected to the mobile unit. Mobile units may also measure characteristics of transmission signals in potential target cells. In a UMTS network, the measurements may be used to determine whether a handover should be performed. Mobile units can perform the measurements for intra-frequency (soft) handover on the current cell and neighbor cells continuously because the base stations all use the same carrier frequency. The signals for different mobile units can be easily distinguished from each other by their different spreading codes.
However, mobile units may need to be tuned to new frequency bands to perform the measurements for inter-frequency or inter-RAT measurements on potential target cells. In many cases artificial transmission gaps may be introduced in the continuous UMTS signals to allow the mobile unit to perform measurements on different frequency bands. For example, the Compressed Mode (CM) has been specified in the 3GPP standards. One basic CM method (named SF/2) allows transmitting the same amount of data over the primary carrier frequency in half of the time by reducing the spreading factor by half, which leaves transmission gaps during the unused time. The mobile unit may therefore tune one or more receivers to other frequency bands and perform the required measurements during the transmission gaps. The mobile units then switch back to the original carrier to continue information transmission.
Compressed mode operations have a number of drawbacks. For example, signals with reduced spreading factors are more susceptible to noise and interference, so the signal power is typically increased during the compressed mode period to maintain signal quality. As a result, the compressed mode, and especially the SF/2 method, can cause significant power variations that result in higher interference levels and network capacity losses. Accordingly, conventional networks attempt to minimize usage of the compressed mode.
One alternative to performing handover measurements in the compressed mode is blind handover. With blind handover, the mobile unit does not perform any measurements on the target cells and therefore the compressed mode is not used. Inter-frequency or inter-RAT handover are triggered only by the mobile unit's measurements performed on the currently used carrier frequency. For example, blind handover might be initiated when the quality of the signals on the current carrier drops below a minimum threshold. However, no measurements will be performed on the target cells (either the other RAT or a different UMTS carrier) and so the best target cell and the signal quality on the new carrier cannot be determined. Target cells for blind handover therefore need to be specified before the blind handover is initiated. For example, a network operator may define a target cell, carrier frequency, and/or RAT for every cell where blind handover is to be used. Usually the most appropriate target cell(s) will be identified during network planning using cell planning data. The mobile unit will then be handed over to the predetermined target cell when blind handover is triggered, e.g., due to bad signal quality on the current carrier.
Blind handover and measurement-based handover algorithms are typically available in the same network and in the same cells at the same time. Thus, a handover algorithm must be selected once the need for an inter-frequency or inter-RAT handover has been identified for a mobile unit, e.g., if the mobile unit drives towards the edge of a coverage area of the serving cell. Quantifying the pros and cons of the different handover algorithms upfront is very difficult, if not impossible, and so conventional algorithm selection techniques implement predefined static selection rules that are conservative or defensive. For example, operators may enable or disable each algorithm separately for certain cells or a network area. For another example, if an operator wishes to restrict compressed mode usage in a certain area, but the mobile unit capabilities indicate that the compressed mode is needed to perform measurements on the target cells, then the operator may instruct the mobile unit to use a blind handover.
Conventional algorithm selection techniques are therefore inflexible and do not take into account the fact that both algorithm alternatives have their benefits, too. The conventional algorithm selection techniques also fail to take into account changing conditions that may affect the selection rules. For example, if the network load is low, then the application of compressed mode may not be an issue and therefore measurement-based may be preferable to blind handovers. On the other hand, blind handover may be advantageous if one appropriate target cell can be identified, especially if the network load is high.
When a blind handover has been selected, the absence of target cell measurements in a blind handover may increase the danger that handover might be directed towards an inappropriate target cell, e.g. if the selection process has not been carried out carefully enough and the best target cells have not been selected. This would have a negative impact on the network performance and could also result in an increased handover failure or call drop rate. The use of fixed target cell definition may also be a drawback in many situations. First, the target cells for inter-RAT or inter-frequency handover need to be specified in advance, which means that the network and/or cell planning for the different network technologies (e.g., UMTS and GSM) needs to be coordinated. For example, GSM target cell(s) must be identified in advance for every UMTS cell. In the case of inter-frequency handover, cell planning for the two frequency layers must be coordinated. Different networks and/or frequency layers are traditionally planned independently, and so the coordination effort might be substantial. Second, the best target cell(s) will not be dynamically updated during the operational phase. However, the network layout will change regularly during the operational phase. Thus, existing cells may not be available at all times and new base stations that provide service to new cells may be installed. It would therefore be necessary to update the target cells for blind handover each time the network layout changes. This is a very time consuming and costly procedure.